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Do You Understand Concept 4.1?
• In considering the origin of cells, why do biologists focus on the origin of the plasma
membrane?
Answer: The origin of cells is linked to the enclosure of biochemical functions within a
membrane that concentrates the reactions and separates them from the environment. The purpose
of cell membrane is to contain the specific stuff needed to support life.
• Figure 4.2 shows cube-shaped representations of cells. Many cells, however, are spherical.
The surface area of a sphere is 4πr2 and the volume is (4/3)(πr3). What is the surface areato-volume ratio of a spherical cell with a diameter of 2 µm and one with a diameter of 3
µm? What are the implications of this difference between the two cells for cell function?
Answer: A sphere with a diameter of 2 µm has a surface area-to-volume ratio of 12.57/4.18 =
3.01. A sphere with a diameter of 3 µm has a surface area-to-volume ratio of 28.27/14.14 = 2.00.
The smaller cell has a larger surface area-to-volume ratio, which allows it to keep pace with
transport though the membrane relative to the metabolic processes within the cell.
• What evolutionary advantages does a eukaryotic cell have compared with a prokaryotic
cell?
Answer: Eukaryotic cells have compartmentalized functions within organelles, providing greater
efficiency and regulation of cell processes, which has allowed more complex cells to evolve.
Do You Understand Concept 4.2?
• Describe the structures that are present in all prokaryotic cells.
Answer: Prokaryotic cells have a cell membrane, a nucleoid where DNA is contained, and
cytoplasm. The cytoplasm consists of cytosol and suspended particles, including ribosomes.
• Compare the structures and functions of bacterial cell walls with those of bacterial
cytoskeletons.
Answer: Both bacterial cell walls and cytoskeletons provide structure and maintain shape. Most
cell walls contain peptidoglycan, which forms a single molecule that surrounds the entire cell.
Another layer, the capsule, may enclose the cell wall and can protect the bacterium from attack
and from drying out. The cytoskeletons are located just inside the plasma membrane and can
extend down the length of the cell to provide shape, such as in rod-shaped bacteria.
• Flagella are made up of proteins that have other functions, and are present in all
prokaryotes. What is the evolutionary advantage of a bacterium that has flagella over
bacteria that do not?
Answer: Flagella provide bacteria with a means of movement and also a means of attachment,
allowing them to move to better in the environment (to acquire food) and colonize other
organisms by binding to them.
Do You Understand Concept 4.3?
• Make a table that summarizes eukaryotic cell organelles with regard to size, numbers per
cell, and functions.
Organelle
Mitochondrion
Nucleus
Endoplasmic
reticulum
Golgi apparatus
Peroxisomes
Lysosomes
Vacuoles
Vesicles
Analogy
Powerplant, engine of the car –
electrical boxes on buildings.
Functions
Convert chemical energy into
ATP, a form the cells can use for
energy.
“brain” – city hall, president,
Location of DNA and DNA
computer chip of car
replication and transcription;
contains the nucleolus
Conveyer belt or assembly line
Rough ER synthesizes proteins
of factory, roads and hwys of a
in ribosomes; smooth ER
city, conduits for water, sewage, modifies molecules; site for lipid
and wires underground
and steroid synthesis
FedEx – processes and packages Protein modification,
the product from ER and shipped concentration, and packaging
within cell or out of cell.
Peroxides are a toxic but natural Accumulates and detoxifies
by-product of a lot reactions –
peroxides
recycling center – break down
waste into harmless substances
Sewage or waste water treatment Hydrolysis of macromolecules
that removes waste from cell.
Storage lockers Storage, catabolism, providing
structure, aiding reproduction
Boxes that contain the presents
Small sacs that come off
Golgi apparatus – contains stuff
• What are some functions of the cell nucleus? What are the advantages of confining these
functions within the nucleus, separated from the cytoplasm?
Answer: The cell nucleus is the location of DNA and site of DNA replication and DNA
transcription to RNA. It also contains the nucleolus, where ribosome assembly begins. The
nucleus provides a separation of the regions where DNA is transcribed and translated, and traffic
of substances between the two spaces can be regulated.
• Compare the structural and functional differences between rough and smooth
endoplasmic reticulum.
Answer: The rough endoplasmic reticulum (RER) appears rough because of ribosomes that
attach during synthesis of proteins that will be modified within the RER for specialized functions
and delivery sites. The smooth endoplasmic reticulum (SER) lacks ribosomes and is more
tubular in shape. It is involved in glycogen degradation, synthesis of lipids and steroids, and
chemical modification of small molecules to make them easier to remove.
• In I-cell disease, an enzyme in the endomembrane system that normally adds
phosphorylated sugar groups to proteins is lacking, and the proteins are not targeted to the
lysosomes as they would be in normal cells. The “I” stands for inclusion bodies that appear
in the cells. What do you think these inclusions are, and why do they accumulate?
Answer: The inclusion bodies are the result of a buildup of proteins, which, because they are not
targeted to the lysosomes in the diseased cells, are not broken down as they normally would be.